Although humans prefer to be active during the day and to sleep at night, diurnal preference ("morningness-eveningness") is highly variable in the population and influenced by many factors. Recently, variations in circadian rhythmicity associated with moderate diurnal preference have been reported. Individuals with extreme diurnal preference (definite morning and evening types) often have an inability to sleep and/or wake at their desired times, and such misalignment between sleep timing and the 24-h social and physical environment can lead to the circadian rhythm sleep disorders Advanced or Delayed Sleep Phase Syndrome (ASPS, DSPS). In the past decade, the genetic basis of circadian rhythmicity has been well-established, including identification of "clock" genes that comprise the molecular mechanism for the generation of circadian rhythms. Because of the relationship between circadian rhythms and diurnal preference, and the established genetic regulation of the circadian system in animals, there is likely to be a genetic basis to extreme diurnal preference and the circadian rhythm sleep disorders. There are reports of associations between diurnal preference and polymorphisms in clock genes, and reports of clock gene alterations in patients with ASPS and DSPS, although not all studies agree. These discrepancies may be due to the phenotyping methods used in those studies. Much remains unknown about the chronobiologic and sleep mechanisms, as well as the genetic basis, of these phenotypes. Here we propose a thorough phenotypic and genetic evaluation of extreme diurnal types to determine the underlying chronobiologic and genetic basis of human diurnal preference, thereby linking specific genes with observable behavior. A careful evaluation of the endogenous circadian phase (Specific Aim 1), period (Specific Aim 2), and pattern of sleep propensity (Specific Aim 3), using well-established methods (constant routine and forced desynchrony protocols) is likely to yield distinct phenotypic groups exhibiting trait-like behaviors? Genetic analysis of extreme diurnal types (Specific Aim 4) may identify associated polymorphisms in clock genes. These data should provide better understanding of the chronobiologic and genetic basis of extreme diurnal preference, leading to better treatments for circadian rhythm sleep disorders.